1. Field
This invention relates to RF induction light sources, and more particularly to the suppression of electromagnetic interference in RF induction light sources.
2. Description of Related Art
The issue of electromagnetic interference (EMI) inflicted by any industrial and consumer product utilizing RF power is the subject of strict domestic and international regulations. According to these regulations, the EMI level emanating from RF light sources must not exceed some threshold value that may interfere with operation surrounding electronic devices, communication, remote control gadgets, medical equipment and life supporting electronics. The permitted EMI level for consumer lighting devices is relaxed at frequencies around 2.65 MHz, but the increase in allowable EMI is limited and EMI still has to be addressed to comply with the regulations.
The conductive EMI of an RF light source (also referred herein as an RF lamp or lamp) is originated by the lamp RF potential Vp on the lamp surface inducing an RF current Ig to the ac line as displacement RF current through the lamp capacitance C to outer space (ground) according to the expression:Ig=Vp2πfCwhere: Vp is the lamp surface RF potential, and f is the lamp driving frequency. The lamp capacitance can be evaluated in the Gaussian system as equal to the lamp effective radius R, C=R in cm or in the CI system as 1.11 R in pF. For an RF lamp size of A19 this capacitance is estimated as about 4 pF; that results in Vp=1 V corresponding to existing regulation limit at 2.65 MHz.
The value of the lamp RF potential Vp is defined by capacitive coupling between RF carrying conductors (mainly the winding of the lamp coupler and associated wire leads) and the lamp re-entrant cavity housing the lamp coupler.
The EMI compliance is especially problematic for integrated, self-ballasted compact RF lamps. The requirements for these compact RF lamps are much stronger, since they are connected to ac line directly through a lamp socket and have no special dedicated contact to the ground, as is the case for powerful RF lamps having remote grounded ballasts.
The effective way to reduce the RF lamp potential is using a bifilar coupler winding consisting of two equal length wire windings wound in parallel, and having their grounded ends on the opposite sides of the coupler.
The essence of this technique is the RF balancing of the coupler with two non-grounded wires on the coupler ends having equal RF potential but opposite phase. Such balancing of the coupler provides the compensation of the opposite phase voltages induced on the re-entrant cavity surface, and thus, on the plasma and the lamp surface.
Although this technique for reduction of conductive EMI has significantly reduced the lamp RF voltage and has been implemented in many commercial RF induction lamps, it appeared that is not enough to comply with the regulation. Some additional means are needed to farther reduce the EMI level to pass the regulations.
A variety of EMI suppression means have been proposed and many of them have been implemented in the market through introduction of RF compact fluorescent lamps, such as a segmented electrostatic shield between the coupler and re-entrant cavity to reduce conductive EMI, a light transparent conductive coating placed between the lamp glass and phosphor, and an external metal conductive coating for lamp partial RF screening.
An alternative (to bifilar winding) way to balance RF coupler has been proposed for RF balancing the coupler by winding on it two wires in the azimuthally opposite directions and to drive such coupler with a symmetrical (push-pull) output ballast. Although the degree of RF compensation in the coupler balancing is expected to be higher than that at bifilar winding, the proposed scheme of compensation has many disadvantages that offset its positive expectation. Probably for this reason, this proposed way of EMI reduction has never been used in commercial products.
The considered above means for EMI reduction are associated with reduction in lamp light output and considerable RF lamp complexity and thus, increased cost.
Another solution of the EMI problem has been proposed that, instead of a complicated shielding of the entire lamp, involves a combination of a bifilar symmetric winding with screening of the RF wire connecting the coupler with ballast by a braided shield. This measure appeared to be enough to pass EMI regulation, yet resulted in considerable gain in lamp efficiency and the lamp simplification.
It would be an advance in the art of EMI reduction of inductive RF fluorescent lamps if one could further improve the EMI shielding at reasonable cost to allow more usage in commercial and residential applications.